Various materials have been reported as electrochromic (hereinafter may be abbreviated as “EC”) materials of which optical absorption properties (colored state and light transmittance) are changed by an electrochemical oxidation-reduction reaction. Metal oxides, such as WO3, are known as inorganic EC materials, but they have a problem in that the method of forming a film thereof is limited to, for example, deposition, which prevents the film from being formed over a large area.
Organic EC materials are described in, for example, PTL 1 disclosing an EC device including an electrically conductive polymer and PTL 2 disclosing an EC device including an organic low-molecular-weight compound such as viologen.
The electrically conductive polymer described in PTL 1 can be directly formed as an EC layer on an electrode by electropolymerization of a monomer. As the electrically conductive polymer that forms the EC layer, for example, polythiophene, polyaniline, and polypyrrole are known. In such electrically conductive polymers, electrochemical oxidation or reduction thereof changes the π-conjugated chain length of the main chain, the electronic state of the highest occupied molecular orbital (HOMO), and the absorption wavelength. These electrically conductive polymers have π-conjugated systems and show absorptions in the visible light region in neutral states. Therefore, they have colors, and the absorption wavelengths shift to the longer wavelength side (infrared region side) by oxidation. The absorption in the visible light region disappears by the shift to the infrared region side, and thereby the EC device loses its color.
On the other hand, in the EC material of the viologen compound described in PTL 2, a dication is dissolved in a solution in a bleached state, and viologen is converted into a radical cation by a reduction reaction to be deposited on an electrode and have a color.
However, these organic EC materials are low in stability and may not be sufficiently bleached even in the bleached states.
In PTL 1, stability is increased by delocalizing the generally unstable radical cation in the molecule. However, the stability is insufficient, which causes a problem of causing deterioration of the material and decrease of the performance by repeating an oxidation-reduction reaction.
Furthermore, the electrically conductive polymer in the neutral state has an absorption band in visible light. Accordingly, if there is a portion in which the electrochemical reaction is insufficient, remnant occurs to make it difficult to obtain high transparency.
In the viologen EC compound of PTL 2, repetition of deposition and dissolution results in a deterioration phenomenon, which is thought to be caused by insolubilization due to irreversible crystallization or polymerization. This deterioration causes “remnant” in which transparency is not obtained even in the bleached state. In addition, the viologen EC compound generates an unstable radical cation when reduced and, thereby, has a problem of being unstable.